It has recently become a matter of importance to understand the molecular basis of frameshift mutation and frameshift suppression in eucaryotes. Current interest in this subject may be attributed primarily to the work of Ames, who has shown that frameshift mutations in Salmonella typhimurium induced by acridine half mustards (ICR compounds) revert at high frequency in the presence of compounds known to be carcinogenic in animals. Using the frequency of reversion of these frameshift mutations as the criterion for mutagenic activity, Ames developed a sensitive test system for screening compounds suspected of being environmental mutagens and therefore harmful to the general public. The major conclusions from this work support the mutation theory of carcinogenesis, since most compounds which are carcinogenic in animals are also mutagenic in Salmonella. It is of interest to determine whether animal carcinogens which are mutagenic in procaryotes are also mutagenic in eucaryotes. The basis for this determination lies in the characterization of frameshift mutations induced by specific compounds in a eucaryotic organism which is amenable to genetic manipulation. The yeast Saccharomyces cerevisiae is ideally suited for this purpose. Evidence from the applicant's postdoctoral work indicates that the acridine half mustard ICR-170 induces frameshift mutations in yeast. Furthermore, frameshift specific suppressors have been isolated by reverting these mutations and the suppressors have been identified in some cases as structural alterations in specific transfer RNA's. Thus, the nature of frameshift mutations in yeast can be deduced by analyzing the nature of suppressor tRNA. Furthermore, the suppressors can be reverted to non-suppressing forms which may result from mutations affecting the regulation and synthesis of tRNA. The experiments described in this proposal represent a continuation of this work and are divided into two inter-related parts: (1) Determination of the molecular mechanism(s) of frameshift mutation and suppression and (2) development of a model genetic system to study tRNA biosynthesis. It is expected that this work will lead to future development of mutagen test systems in eucaryotes and should also further our understanding of gene regulation.